Add integration tests

Integration tests that test the functionality of each endpoint should be included in the CI pipeline to minimize the probability that merges do not introduce regression.

See [approach in FOCA](https://github.com/elixir-cloud-aai/foca/blob/cf8a3144ad24902c25eea43008c5e28083e3b952/.travis.yml#L30) for an example on how these tests can be implemented elegantly.